This invention relates to semiconductor memory devices and, more particularly, the present invention relates to devices that utilize magnetic thin films
Memory devices are an extremely important component in electronic systems. The three most important commercial high-density memory technologies are SRAM, DRAM, and FLASH. Each of these memory devices uses an electronic charge to store information and each has its own advantages. For example, SRAM has fast read and write speeds, but it is volatile and requires a large cell area. DRAM has a high memory density, but it is also volatile and requires a refresh of the storage capacitor every few milliseconds. This refresh requirement increases the complexity of the control electronics.
FLASH is the major nonvolatile memory device in use today. Typical FLASH memory devices use charges trapped in a floating oxide layer to store information. Drawbacks to FLASH include high voltage requirements and slow program and erase times. Also, FLASH memory has a poor write endurance of about 104-106 cycles before memory failure. In addition, to maintain reasonable data retention, the thickness of the gate oxide has to stay above the threshold that allows electron tunneling. This thickness requirement severely restricts the scaling trends of FLASH memory.
To overcome these shortcomings, magnetic memory devices are being evaluated. One such device is magnetoresistive random access memory (hereinafter referred to as xe2x80x9cMRAMxe2x80x9d). MRAM has the potential to have a speed performance similar to DRAM. To be commercially viable, however, MRAM should have comparable memory density to current memory technologies, be scalable for future generations, operate at low voltages, have low power consumption, and have competitive read/write speeds.
For an MRAM device, the stability of the memory state, the repeatability of the read/write cycles, and the power consumption are some of the more important aspects of its design characteristics. A memory state in MRAM is not maintained by power, but rather by the direction of a magnetic moment vector. In typical MRAM devices, storing data is accomplished by applying magnetic fields and causing a magnetic material in an MRAM cell to be magnetized into either of two possible memory states. Recalling data is accomplished by sensing the resistive state of the cell which depends on the magnetic state. The magnetic fields are created by passing currents through strip lines external to the magnetic structure or through the magnetic structures themselves.
For standard MRAM devices, the switching field Hsw is proportional to the total anisotropy Hkxe2x80x94total of the bit, which can include contributions from the device shape and material composition. Most MRAM devices rely on a bit shape having an aspect ratio greater than unity to create a shape anisotropy Hkxe2x80x94shape that provides the switching field Hsw.
However, there are several drawbacks to relying on Hkxe2x80x94shape to provide Hsw. First, Hkxe2x80x94shape increases as the bit dimension shrinks so that Hsw increases for a given shape and film thickness. A bit with larger Hsw requires more current to switch. Second, variations in Hsw will occur due to variations in bit shape from lithographic patterning and etching. These variations will increase as the bit size shrinks due to the finite resolution of optical lithography and etch processes. Variations in Hsw translate into a smaller operating window for programming of the bits and are therefore undesirable. Third, the range over which the magnitude of Hkxe2x80x94shape can be varied is limited. Only certain bit shapes produce reliable switching and although varying the thickness of the film will vary Hkxe2x80x94shape, there is a maximum bit thickness above which the bit switching quality degrades due to domain formation.
Other MRAM devices rely on anisotropy from pair ordering of like atoms to provide all or part of Hkxe2x80x94total. For example, if a nickel iron (NiFe) film is deposited in a magnetic field, a small percentage of the iron (Fe) and nickel (Ni) atoms pair with like atoms and form chains parallel to the magnetic field, providing a pair anisotropy of approximately 5 Oe substantially parallel to the magnetic field direction.
Pair ordering anisotropy Hkxe2x80x94pair has the advantage of being substantially independent of bit shape and is relatively unchanged as the bit size decreases. However, the magnitude and direction of Hkpair can drift with temperature. This temperature drift substantially results from thermal diffusion of the atom pairs. In addition, the magnitude of Hkpair is predominately fixed for a particular magnetic material which limits the range of Hsw.
Based on the limitations of shape and pair anisotropy discussed above, it is clear that a need exists to adjust the switching magnetic field Hsw using a magnetic anisotropy which is adjustable over a wide range, does not substantially depend on bit shape, and is thermally stable. Accordingly, it is an object of the present invention to provide a new and improved method of fabricating a magnetoresistive random access memory device.